London, Oct. 3: It is the biggest-selling adult fiction book of all time and has earned its author a reputed '140 million with its plot about a global conspiracy to suppress Christ's marriage.

The Da Vinci Code has sold more than 12 million copies and has been translated into 42 languages. But now two writers are suing its publishers, claiming that it was copied from their best-seller that first appeared over 20 years ago.

Michael Baigent and Richard Leigh claim that Dan Brown, the 39-year-old former English teacher from New Hampshire, has 'lifted the whole architecture' of the research that they carried out for their non-fiction work The Holy Blood and the Holy Grail, which they co-wrote with Henry Lincoln.

They claim that the similarities between the two books are such that they have no choice but to sue Random House, whose imprint Doubleday is the publisher of Brown's novel.

Leigh, after issuing the writ, said: 'It's not that Dan Brown has lifted certain ideas because a number of people have done that before. It's rather that he's lifted the whole architecture ' the whole jigsaw puzzle ' and hung it on to the peg of a fictional thriller.'

The Da Vinci Code tells the story of a Harvard professor who stumbles across a conspiracy to suppress Jesus Christ's marriage to Mary Magdalene and his fathering of a royal bloodline.

Baigent and Leigh claim that the novel's premise and chunks of factual research are plagiarised from their original historical hypothesis, which has sold more than two million copies despite being denounced by several Church commentators as 'pseudo-history'.

Baigent said: 'Whether our hypothesis is right or wrong is irrelevant. The fact is that this is work that we put together and spent years and years building up.'

The Holy Blood and theHoly Grail was based on six years of research and hypotheses that Jesus married Mary Magdalene and founded a royal bloodline protected by a series of esoteric societies including the Knights Templar and the Priory of Sion, one of whose 'Grand Master' heads is claimed to have been Leonardo da Vinci.

The authors argue that Brown lifted their all-important list of the Grand Masters, who supposedly guarded the secret documents pertaining to Christ's bloodline, without acknowledgement.

The only mention of their book is when the villain of The Da Vinci Code, an eccentric English historian called Sir Leigh Teabing, lifts a copy off his bookshelf and says: 'To my taste, the authors made some dubious leaps of faith in their analysis, but their fundamental premise is sound.'

The name Leigh Teabing is an anagram of Leigh and Baigent, the authors point out, while his physical description ' he walks with the aid of crutches ' is allegedly based on the third author, Henry Lincoln, who walks with a limp.

Lincoln has decided not to be part of the copyright action because of ill health, but is said to support it.